The invention concerns power tools having a multicomponent transmission including at least one speed reducing bevel gear set, interposed between a driving motor and an output member. More particularly, the invention concerns a thrust bearing arrangement for a crown wheel in a tool of this type with a reciprocating output, such as a reciprocating power saw.
Reciprocating power saws typically use a relatively simple transmission with a bevel gear set input in which a pinion drives a crown wheel for a one stage speed reduction. An eccentrically mounted pin on the crown wheel engages some form of yoke connected to a saw blade carrier extension for converting the input rotary motion to a reciprocating output. In hand held reciprocating saws it is particularly desirable to minimize overall weight and bulk and especially to achieve compactness of the transmission. In use, the transmission housing is generally used as a forward hand-grip or hold.
Knight (U.S. Pat. No. 3,657,813), in a tree pruning saw, exemplifies the transmission arrangement outlined above but in a relatively deep configuration (as measured from the top of the saw blade carrier extension to the base of the crown wheel mounting). Part of this depth comes from a radial bearing arrangement for the crown wheel which straddles the crown wheel to provide a stable mount for it at the expense of compactness of the transmission overall.
In the motion-converting hand drill attachment of Dodegge (U.S. Pat. No. 2,713,271), the connection from the crown wheel eccentric pin to the saw blade carrier extension yoke is much more compact (in depth). But, although Dodegge's disclosure lacks details., it appears that, again, a radial bearing configuration of substantial axial extent is relied on for crown wheel stability so that the transmission remains relatively deep or bulky. It appears also that thrust loads imposed in crown wheel 91 by pinion 103 are absorbed by boss 94 of the housing H.
O'Banion (U.S. Pat. No. 4,385,443) and Moores (U.S. Pat. No. 4,550,501), both disclose hand-held reciprocating power saws of similar configuration. In particular, to minimize depth in the forward hand grip area (which is also the transmission housing), both have adopted, for the crown wheel, a radial bearing of limited axial extent and a simple thrust bearing comprising a "cap" retained on the upper end of the fixed crown wheel bearing spindle by a single retaining screw. This may be called an open bevel gear case arrangement in that the crown wheel spindle or journal and its thrust bearing are cantilevered from a gear case frame member below the crown wheel and there is no frame or support structure above the top or back face of the crown wheel. For transferring thrust load from the crown wheel to say, the frame or housing of the transmission there is no obvious compact alternative to this cantilevered arrangement, given that the saw blade carrier or its extension reciprocates closely above the crown wheel. Use of a live spindle for the crown wheel and placing a thrust bearing at its lower end would again undesirably deepen the transmission in the hand grip area.
In the conventional cantilevered thrust bearing arrangement of O'Banion and Moores, the diameter of the thrust cap is necessarily limited by the presence of the eccentrically mounted drive pin extending upwardly from the back face of the crown wheel. The limited dimensions of the crown wheel bearing surfaces, both radial and thrust, in relation to crown wheel diameter make stability of the crown wheel uncertain. Even before wear has occurred an accumulalation of manufacturing tolerances can result in looseness allowing the crown wheel to rock, affecting gear mesh and concentrating loadings in the bearing surfaces and the retaining screw to such an extent &hat rapid wear occurs and durability of the saws in heavy duty applications is not always acceptable.